Field of the Invention
The present invention relates to the control of grid frequency in an electric power grid.
Description of the Related Technology
Since the standardization of the frequency of alternating current (AC) electricity in large scale electric power grids in the mid-20th century around the globe, consumers of electricity have been able to enjoy a consistent and dependable service of electricity, ensuring safe and reproducible use of electrical appliances. For example, in the UK, the standardized nominal grid frequency is set at 50 Hz. To promote the reliability of the grid frequency, grid operators provide ranges of frequency around the nominal grid frequency outside which they may ensure service providers or consumers the grid frequency will not stray. For example, using the nominal UK grid frequency of 50 Hz, the grid operator may aim that the grid frequency should not go outside the range 50±0.5 Hz (or ±1% of the nominal frequency).
The grid frequency depends strongly on the frequency of electricity produced by generators of electrical power attached to the grid. Large capacity generators may, for example, consist of a driven mass incorporating magnetic poles rotating inside a wire coil. This mass may be driven, for example, by steam acting on a turbine, where the steam is produced, for example, by the burning of fossil fuels. Taking into account the number of poles associated with a generator, the frequency of electricity produced is proportional to the rotation speed of the generator. For example, a generator with 6 poles rotating at 1000 RPM would produce electricity with a frequency of 50 Hz. In other examples of power generation, for example those which generate direct current (DC) electricity, such as solar panels, inverters may be employed to provide AC electricity at a certain frequency, for example the nominal grid frequency, to the grid.
In a grid where the provision and consumption of electrical power is balanced, the rotation speed of the generators, for example, can be set so as to effect, precisely, a grid frequency at the nominal grid frequency. However, if there is a change in the power balance, for example a sudden increase in demand, the rotational speed of a generator responding to that change, for a given drive of the turbine for example, may reduce. As a result, the frequency of the generated electricity may reduce, and hence the grid frequency may reduce. This situation may be rectified by applying a greater drive to the turbine of the generator, but this may take considerable time, or, in the case a generator is working at full capacity, may not be possible. A sudden power imbalance may also arise, for example, when a power station, or interconnector is suddenly lost from the grid. In the case of a sudden reduced demand of power, a generator may reduce its power output in order to return the grid frequency to the nominal frequency accordingly. Equally as a result of a decrease in demand, or for example, if an interconnector is exporting power from the grid when it is lost, both of which affect the power balance, there may be an increase in the rotational speed of a generator and hence cause an increase in the frequency of generated electricity and hence grid frequency.
An existing method of addressing undesirable frequency changes is to run generators at reduced capacity, for example, the power output of a generator may be set at 95% of the total output capacity of the generator. If there is, for example, a change in power balance, resulting in a change in grid frequency, these generators can respond by providing, within a few or a few tens of seconds, an increased or reduced power output accordingly.
It is uneconomical, however, to run generation units at reduced power output level in anticipation of events which may be relatively seldom. Moreover, the speed at which generators are able to provide a response may not be sufficient to keep the grid frequency within the specified range, for example, for particularly sudden events, for events involving particularly large changes in frequency, or for events occurring in relatively small grids such as those associated with island nations such as the UK or New Zealand. The response provided within the first second or so after the sudden change can have the largest impact on reducing the negative effects associated with a deviation of the grid frequency from the nominal value, for example, damage to large electrical machines.
A further method of addressing changes in grid frequency is to arrange for devices, such as user appliances, to monitor grid frequency at their location and to react to changes in same according to predetermined criteria. For example, in response to a sudden drop in frequency, consumption of electrical power may be reduced in order to counteract against such changes. WO2011085477A1 provides an example of such a method. However, these approaches are inflexible as they are limited to effecting a local response to a local change in power balance. Moreover the collective response that a number of locally implemented responses provides is uncertain, and may, for example, lead to a collective over-response.
It is an object of embodiments of the present invention described herein to at least mitigate one or more problems of the prior art.